The first one, obviously, is Improved Succubus, which has been modified in the Wrath Beta to confer up to a 100% resistance to interruption caused by damage while channeling Seduction. Succubi have four basic abilities, although they are famous this one alone. If a Succubus is ever brought out for PvP, it is purely for Seduction. They are the easiest demon to kill next to an Imp out of Phase Shift and thus almost never see Arena or even Battleground play. However, Succubi provide invaluable crowd control for humanoid-heavy instances. It helps to use an AddOn like FocusFrame to keep an eye on the Succubus' target because learning to reapply Seduction -- try to key bind it -- is crucial to keeping her alive.
Seduction is a great ability but requires micro-management to use well (though it's odd that a channeled spell should have a cast time...). Succubi also have Lash of Pain, its basic melee attack that deals Shadow damage. With the removal of Improved Lash of Pain, it now puts out significantly lower DPS (despite Unholy Power) and leans more towards utility. If you can manage to keep Soothing Kiss on a melee target without the Succubus dying, it's a useful debuff. Lastly, its Lesser Invisibility works well to keep it alive before engaging targets, particularly in PvP. If it's out of combat, keep it invisible. Note that invisibility is not stealth, and she cannot be revealed by stealth detection mechanics.
I like to think of the Succubus as a finesse pet. Its fragility requires close attention in PvP and Seduction is a channeled spell that doesn't last as long as, say, Polymorph or Freezing Trap, requiring frequent reapplication. Even with the new pet scaling, Succubi are still vulnerable pets. The good news is that Demonology Warlocks get an additional tool to keep the Succubus up in Wrath with Demonic Empowerment, effectively a Rogue's Vanish on a 1-minute timer.
When sacrificed to Demonic Sacrifice, a Succubus provides a 10% increase to all Shadow damage done. Whereas it didn't make much sense before with Soul Link active, it's actually an option now considering that Soul Link no longer confers additional damage for master and pet. Soul Link with a Succubus -- in PvP, at least -- is very bad idea to begin with, anyway. Altogether not too many changes in Wrath, but the few new and reworked Demonology talents keep playing with a Succubus challenging and fun.